Multiplex



June 17, 1924. '1,497,715

L. ESPENSCHIED l MULTI FLEX SIGNALING CIRCUITS zum alf/w114 WM,

ATTOR N EY June 17 1924-.r

L. ESPENSCHIED MULTIPLEX S IGNALING CIRCUITS Filed July l, 192] 2 Sheets-Sheet 2 INVENTOR f/wwczed ATTORNEY Patented June 17, 1924.

UNITED STATES PATENT OFFICE.

LLOYD ESPENSCHIED, OF QUEENS, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMYANY, A CORPORATION OF NEW YORK.

Application led July 1,

tems, and more particularly to signal-ing` systems of the multiplex type.

Heretofore,.in multiplex signaling circuits employing carrier currents for transmission over individual channels, it, has been customary to select into the individual circuits corresponding to the channels of the multiplex system by simultaneously applying all of the frequencies correspond-A ing to the different channels upon a selectingA system for filtering individual frequenciesl The separation` into the proper channels. into thel different channels is accomplished directly at the frequencies of transmission, without the intervention of' any frequency conversion process.

In accordance with the present invention, the selecting process is carried out in connection with the process of frequency conversion. In carrying out the process at the receiving portion of the system, several signaling bands are received upon a common circuit and the lowest band selected from the remaining bands for transmission to its proper channel. The remaining bands are then stepped down together. so that the neXt lowest band will occupy substantially the saine frequency7 range as the band. previously selected. This band is then selected from the bands above and transmitted to itschannel. rlhis process is continued until all of the signaling bands have been select-ed` into the proper circuits. In accordance with'this method` the terminal. channel circuits are all designed to have the same cut-oflrharacteristic and the received transmission band carrying the plurality of individual hands is stepped Yinto the channels one at a time through a succession of frequency conversions. At the transmitting part of the system, the method constitutes superimposing the severalfbands by taking an individual band, stepping it up to a frequency just above the frequency range normally occupied by the second band. combining with i the stepped up hand a second band, stepping 1921. Serial No. 481,915.

quency range just above that normally occupied by a third band, combining a third band with the stepped up bands, etc., for as many channels as are to be combined. -This method possesses the advantage that the act of selecting each carrier channel to or from its individual terminal circuit is carried out always at a frequency which is most favorable to the selecting action, namely the lowest frequency channel ofthe carrier group.

The invention may now be more fully understood from the following description, when read. in connection with the accompanying drawings, Figures l and 2 of which illustrate two different forms of circuit arrangements for carrying out the principles of the invention.

Referring to Fig. l, ML designates a main transmission line over-which carrier currents may be transmitted, and L L and L, low frequency signaling lines. The line L1 is balanced by a network Nl and is associated with the transmitting channel TL1 and receiving channel RL1 through the usual hybrid coil 11. The transmitting channel TLl leads to the input side of a modulator Ma which may be of any well-known type, but as illustrated is a vacuum tube modula-l tor. The receiving channel RL. leads from the output circuit of a dcmodulator or detector D1. which is also illustrated as a vacuum tube detector. although any wellknown type of detector maybe employed. Selective arrangements such as filters, are employed in the transmitting and receiving channels. In the case illustrated, it is assumed that the signaling band occupies a range from zero to 3.000 cycles. and, therefore. a low pass filter RF, and a low pass filter TF, are included in the receiving and transmitting channels, respectively. the upper cut-ofi" frequency of the filters being in the neighborhood ofA 3,000 cycles. The modulator1 M.I and the detector D, are each illustrated as being supplied with a carrier frequency of 3.000 cycles, although the particular frequency employed is immaterial, except that it should be at or above the upper limit of the low frequency signaling band.

The output side of the modulator M1 is associated with the input side of a modulator M2 through a circuit including a band Cil filter TF2 arranged to transmit a band of frequencies between 3,000 and 6,000 cycles. The input side of the detector D1 is similarly associated with the output side of a detector D2 through a circuit including a band filter BF2 having the same transmission limits. A second low frequency signaling circuit L2 is balanced by a network N2 and is associated With a transmitting channel TL2 and a receiving channel HL2 through a hybrid coil 12. Low pass filters TF2 and BF2 are included in these channels, the low pass filters being illustrated as having the same upper cut-off frequency as the low pass vfilters previously described, this limit being set upon the assumption that thelovv frequency vsignaling band to be transmitted from the line L2 will be the same as that transmitted from the line L1.

V i,[tis not essential that these limits should Vbe used, however, the only requirement be- Vthat the low pass filters have their cutn ,olfI frequencies above the loiv frequency signaling band and below the stepped up band `ivith Which they are to be combined. The channel TL2 leads to the input circuit of the modulator M2 and the receiving channel RL2 leads from the output side of the demodulator D2. The demodulator D2 and modulator M2 are also supplied .with a carrier frequency which is illustrated as being of 3,000 cycles, although some discretion Pmay be exercised in determining the carrier frequency. f

,I The output circuit of the modulator M2 leads to a common transmitting channel TL and the common receiving channel RL leads to theinput circuit of the detector D2. The common Vcircuits TL and RL are associated Withthe main line ML through a hybrid coil 10. High pass filters TF3 and RFS are included in the connnon circuits TL and RL adjacent to the modulator M., and the detector D2, respectively. The loiver culi-ofil oint of theseI filters is indicated as being 9,000 cycles, although the frequency so employed will varywith circumstances.

JThe loiv frequency transmission line Ls is balanced by a network N2 and is associated by means kof a hybrid coil i3 with a transmitting channel TL.and a receiving channelllLB. Loiv pass Ifilters TF, and

,RFM similar to those already` described, are

included in these channels. The circuit TL3 vis connected to thecommon transmitting circuit TL and .the circuit HL2 .is connected With the common receiving circuit RL. y yFurther details `of the apparatus will bel 'unvrdierstood from the description .of the op- In considering theoperation, the

columns of numbers placed above and below "f ,cir'zuit Aindicate the frequencies involved at, 't i 63 individual portions of the apparatus in the se particular .points in the circuits, and

these frequencies shouldy be kept in mind 'when reading the description of the operation, which is as follows'. Y

The low frequency signaling band incoming from the line Tg is transmitted into the channel TL, through the filter TF, 'to the input circuit of the modulator M1. Assuming that this band extends from zero to 3,000 cycles, it combinesk in the modulator M, with the carrier frequency of 3,000 cycles to produce upper and lower side bands and a frequency component corresponding to the carrier as indicated in the column immediately below the modulator M1. The uppei` side band Will entend from 3,000 to 6,000 cycles, and by means of the filter TF2', the .lower side band and, if desired, the carrier, may be suppressed. (See numbers included in the dotted rectangle of the column below the modulator M1.) The selected band from 3,000 to 6,000 cycles, being passed through the filter TF2', is combined with a low frequency band from the line L2, Which is ltransmitied through the low pass filter TF2. This band may also occupy the range from Zero to 3,000 cycles.

The complete band of frequencies to be impressed upon the modulator M2 is indicated in the column just below the input side of said niiodulator. As illustrated, this complete band comprises [tivo sub-bands, one numbered l and comprising frequencies from 3,000 to 6,000 cycles corresponding to the signal incoming from the line L2, and the other designated 2 and comprising frequencies from Zero to 3,000 cycles, corresponding to signals incoming from vthe line L2. This band of frequencies now modulates with the carrier frequency of 3,000 cj, ties applied to the modulator' M2 to produce upper and lower side frequencies as Well as a frequency corresponding `to the carrier, as indicated in the column beloiv the output side of the modulator M2. The portions of each band corresponding to the original sub-bands are indicated by the numerals l and 2. As the filter TF, in the output circuit of the modulator has its lower cut-off point at 3,000 cycles, the carrier frequency and the loiver side band frequencies will be suppressed, and tivo subbands of 6,000 to 9,000 cycles and 3,000 lo 0,000 cycles, respectively, corresponding to the signals from the lines L1 and L2 will be transmittedto the common transmitting circuit TL. A. low. frequency signaling band from the line L3 will also pass into the transmitting (manuel TL, through the filter TE, an d be combined in the circuit ,TL with the two sub-bands previously referred to. Consequently, frequencies .from Zero` to 9,000 cycles lwill appearA in the circuit TL and will be transmitted le the main line ML. These frequencies comprise sub-bands .cor-

rsponding to the signals from the three cated by the numbers in the column immediately belorv the common circuit TL.

Tn receiving` assuming that an operation similar to that `inst described is proceeding at a distant transmitting station, frequencies from Zero to 9,000 cycles incoming from the line il'lll Will be transmitted to the com* mon receiving circuit RL. These frequen-v cies. as indicated by the column above thc circuit BL.v comprise three sub-bands numbered l, 2 and 3, corresponding` to the three lov.' frequency transmission circuits to which .lie signals they represent will ultimately be transmitted. The loiver band from Zero to l 3.000 cycles vvill be passed by the low pass filter RFR into the channel RLH, being eX eluded from the detector by the high pass filter EFX. The. signaling band passed b the filter RFg will be transmitted through the hybrid coil to the line L?. As this band of frequencies is supposed to represent a. signal in the loir frequency state, no detecting operation is necessary with respect thereto.

The high pass filter BF... selects the tivo sub-bands numbered 1 and 2, and impresses these bands upon the input side of the detector DE, Where they are modulated With the carrier frequency of 3,000 cycles to produce upper and lower side bands and carrier frequency component indicated in the column designated Output D2. BF2 is lonY pass filter, selecting frequencies from Zero to 53.000 cycles. and the filter RFE is a band filter selecting frequencies from 3,000 to 6,000 cycles. frequencies above 6,000 cycles will be eliminated, as indicated by the frequencies included in the dotted rectangle in the column last referred to. The band of frequencies from zero to 3.000 cycles` designated by the numeral 2. corresponds to a low frequency signal for transmission to the line L2, and this band of frequencies. being selected by the filter RF,... is transmitted through the hybrid coil l2 to the low frequency line. The remaining band of 3,000 to 6.000 cycles is passed through the filter BF, and modulated with the frequency of 3,000 cycles in the deuiodulntor D., to produce upper and lower side bands and carrier, as indicated by the column of figures marked Output DI. The louvv pass filter RF, selects the lower hand which is transmitted through the hybrid coil ll to the low frequency line.

The system as above desc ibed is limited to not more than three channelsr` Where the same carrier frequency is employed throughout. This is for the reason that the carrier frequency lies with in the range of the complete band transmitted from the main line ML, so that when a. fourth channel is ernployed.y the band of frequencies from 9,000 to 12,000 cycles, which Would then be in volved in transmission over the main line As the filter.

ML, would, in modulating with the carrier frequency of 3,000 cycles, produce a lower side band lying Within the saine range as the upper side band produced by the band eX- tending from 3000 to 6,000 cycles. interference between the channels would consequently result. Tn order that more channels may be employed, it is necessary to adopt a slightly different principle for the additional channels. This principle involves using carrier frequencies for the additional modulators and detectors necessitated by the addi.- tional channels, which frequencies Will be higher than the highest frequency of the total band involved at the additional modulati and demodulating points, The principle employed will be clear from Fig. 2, in which the apparatus associated With the lines L1, L2 and la, will be identical with the corresponding apparatus in Fig. l, .vith the exception that the filters RE,l and TF2 will he band lters having ranges from 3,000 to 0,000 cycles, instead of being high pass filters. This particular range is used for purposes of illustration. it being understood that the range will vary depending upon the number and Width of signaling bands employed. The channels TL, and BLR, instead of being connected directly to the common transmitting and receiving channels TL and BL, are connected to the input side of the modulator M3 and the output side of the demodulator D3. respectively. The output side of the modulator MY is connected to the input side of the modulator lrl,L through a band filter TF,l Whose range is indicated as being from 3,000 to 12.000 cycles. Similarly, the input side of the detector D., is connected through a band filter lill having the saine frequency range, to the output side of the demodulator 1),. A loir frequency transmission line L,i is balanced by a net Work N, and is associated through a hybrid coil 14 With a transmitting channel TLL, and a receiving channel RL4 including` lovv pass filters TF4 and R141, respectively. The channels TL, and RL4 conneet the line L, to the input side of the modulator hl, and output side of the deinodulater D., respectively.

The output circuit of the modulator M4 is connected to the common transmitting cir cuit TL through a band filter TFJ having a` transmission range indicated as extending from 3.000 to 15.000 cycles. Similarly, the input circuit of the dcmodulator D1 is connected With the common receiving channel RL through a selecting filter lill. This filter may be either a band filter similar to the fil-ter TF5 or it 'may be a high pass filter as indicated. A lon7 frequency line L5 is balanced by a network N5 and is connected through a hybrid coil l5 with transmitting channel TLl` and receiving channel RL, these channels including low Y pass filters TF,S and RFS, respectively, and being connected directly to the common transmitting and receiving' circuits TL and RL, respectively. lf additional low frequency lines are to be included in the system, the receiving and transmitting channels of the line L5 will be connected to additional demodulators and detectors.

The modulator M, and the detector D3 are indicated as being supplied with a carrier frequency of 12,000 cycles, and simiiarly, the modulator M and detector D, are indicated as being supplied with a carrier frequency of 15,000 cycles, These frequencies must exceed the upper limit of the impressed band by at least the width of next incoming signal band impressed upon each particular deniodulator and detector. ln the case of' the modulator M, the band impressed upon the inputcircuit of the modulator extends from Zero to 9,000 cycles, and by modulating with the carrier of 12,000 cycles, will produce an invertedlower side band extending from 3,000 to 12,000 cycles, so that the band of frequencies will be elevated in the frequency spectrum a sufiicient amount to permit another signaling band to be inserted below the modulated band. The band of frequencies impressed upon the-detector D3 extends from 3,000 to 12,000 cycles, and by modulating with the carrier frequency of 12,000 cycles, an inverted lower side band will be produced extending from zero to 9,000 cycles. In this instance, the inversion of the lower side bands restores the frequencies to their normal relation as the input Vfrequency band will be already inverted before being applied to the detector, as will be clear later. The lower side band of the detected side band will, therefore, represent the low frequency signal and may be selected directly to the low frequency line.

Similarly, in the case of the modulator MMthe input band extends from zero to 12,000 cycles, and this band` in modulating with the carrier frequency of 15,000 cycles,

will be stepped up to a range from 3,000 to 15,000 cycles to permit an additional frequency band to be inserted below the stepped-up band. Likewise the band impressed upon the detector D4 extends from 3,000 to 15,000 cycles, and by modulating with the carrier of 15,000 cycles, this is stepped down into a range extending from zero up to 12,000 cycles, thereby permitting the direct selection of the lower frequencies of this band into a low frequency signaling circuit.

The principles of this form of the invention and the details of the apparatus will be more clearly understood from the description of the operation, which is as follows:

A low frequency signaling band extending from zero to 3,000 cycles incoming from the line Lj is selected into the channel TL1 through the filter TF1 and applied to the modulator M1. The operation of successive modulations in combination with additional low frequency bands from the lines L2 and L, is the same as described in connection with Fig. 1, so that a band is finally selected to be applied to the input circuit of the modulator M3 which extends from zero to 9,000 cycles and includes three sub-bands numbered 1, 2, and 3, respectively, these subbands corresponding to the lines L LJ rand L. This entire band of frequencies is then impressed upon the modulator M, and modulates with the carrier frequency of 12,000 cycles to produce the upper and lower side bands and carrier frequency component indicated bythe column of figures designated Output MQ. The filter TF4 eliminates all frequencies above 12,000 cycles so that the upper side band is supijiressed.v The lower side band now extends from 3,000 to 12,000 cycles.. .lt will be understood, however, that the frequencies in this band will be reversed with respect to the frequencies in the input bandso that the position of the sub-bands numbered 1, 2 and 3 in the frequency spectrum will be reversed. rand the band numbered 3, which on the input 95 side of the modulator was the lowest band of the series, is now ythe highest band of the series. The individual frequencies in each of these sub-bands are also reversed in order, so that the highest frequency of the 100 sub-band is now the lowest frequency of the sub-band, and vice versa. The lower side band of from 3,000 to 12,000 cycles selected bythe filter TF4 is combined with a signaling band from the line'L, said signal- 105 ing band extending, in the case illustrated, from Zero to 3,000 cyclesl The total band comprising the four sub-bands will now involve the limiting frequencies indicated in the column marked Input M4.

This range of frequencies,` upon being applied to the modulator M4, modulates with the carrier frequency of 15,000 cycles to produce upper and lower side bands as indicated in the column marked Output BL. 115 The filter TF5 suppresses all 'frequencies above 15,000 cycles so that only the lower side band remains. This lower side band includes four sub-bands whose relative positions in the spectrum have been reversed 120 as regards the four sub-bands applied to the input circuit of the modulator. For example, the sub-band designated 4 is now the highest range of frequencies Whereas before it was the lowest, and the sub-band 125 numbered 3 which was the highest sub-band before modulation is now the lowest subband in the group. Sub-bands 1, 2 and 3 now have their individual frequencies re-` versed With respect to the input hands of the 13 modulator M4, but their individual frequencies are in the same relative order in the frequency spectrum as they originally occupied in the corresponding sub-bands applied to the input circuit of the modulator M3. The individual frequencies of these three subbands are consequently not inverted with respect to the corresponding frequencies transmitted from the low frequency lines. The individual frequencies of the four sub-bands, however, are inverted with respect to the component frequencies of the band incoming from the line L4.

The band of frequencies selected by the filter TF 5 .which has been shifted in the frequency spectrum by 3,000 cycles, is now combined with a signaling band from the line Lg, which, in the case illustrated, is assumed to extend from zero to 3,000 cycles and is passed through the filter TF5 of the channel TL3 so that a combined band including five sub-bands extending from Zero to 15,000 cycles is finally transmitted to the main line ML. Of the five sub-bands transmitted, bands l, 2, 3 and 4 have all been shifted in the frequency spectrum but the fifth band is unshifted. The component frequencies of bands l., 2, 3 and 5, are not inverted, while the component frequencies of the fourth band are inverted with respect to the original frequencies incoming from the low frequency lines.

rssuming that a similar transmitting operation is taking place at distant sending station., a similar range of frequencies comes in through the line ML and passes into the common receiving circuit RL. This range of frequencies extends from aero to 15,000 cycles and, comprises .five sub-bands having frequency limits as i Adicated in the column designated From line. The fifth subba-nd, comprising freruiencies from zero to 3,000 cycles will be selected without demodulation by means of the low pass filter RF5 and transmitted to the line L5. The remaining four sub-bands will be passed by the high pass lter to the input circuit of the detector D4. These frequencies, by demodulation with a carrier frequency of 15,000 cycles, produce in the output circuit of the detector D upper and lowerl side bands and carrier componentas indicated by the column of figures marked Gutput Dj. Since the filters RF 4 und RFA, discriminate against all frequencies above 12,000 cycles, the carrier component and the upper side band will be suppressed, leaving a lower side band having four sub-bands designated 1,2, 3 and 4. It will be noted that of the four sub-bands impressed upon the demodulator, the frequencies in sub-bands l, 2 and 3 are not inverted with respect to the signaling frequencies of the bands from which .they originated,` while thev frequencies in spect to the 'original frequencies. The frequency in the individual sub-bands of the lower side band of the demodulator D4 are inverted with respect to the input frequencies and the order of the bands in the frequency spectrum is 'also inverted. Consequently, the fourth sub-band which was the highest sub-band on the input sideof the demodulator becomes the lowest sub-bandl in the output, and its frequency range is from zero to 3,000 cycles. Therefore, this band of frequencies will be selected by the filter RF 4 and will be transmitted directly to the line L, as a signal.

The remaining sub-bands fall within the limits of the filter RF; and are impressed upon the demodulator D3. By demodulation with the carrier frequency of 12,000 cycles supplied to "the demo'dulator, the two side bands and carrier component indicated in the column designated Output D3 will be suppressed. The filters RF3 and RFS suppress all frequencies above 9,000 cycles; Consequently, the carrier component and the upper side band are eliminated. The three sub-bands making up the lower side band are again inverted in their order, and their frequencies are also inverted, with the consequence that the relative order of the frequencies in each band will correspond to the order ofthe frequencies in the original bands, and the bands will be arranged in the spectrum in the inverse numerical. order of the bands. The band designated 3, which extends from zero to 3,000 cycles, will, therefore, be selected by the filter RF3 and transmitted directly to the line L3 as a signal, while the remaining two bands will be passed through the filter RFE. tothe detector D2. From this point the operation will be as` described in connection with Fig. l.

It will be understood that the frequencies hereinbefore referred to and indicated on the drawing have merely been given in order to clearly elucidate the principles 0f the invention, though in practice other frepiencies than those given may be used. For example, the carrier frequency applied to the modulator M1 and dcmodulator D1 may be 6,000 cycles and the carrier frequency applied to the modulator M2 and the demodulator D, may be 9,000 cycles. In this case, the band applied to the modulator Ml will produce in the output circuit a lower side baud from 3.000 to 6,000 cycles, which will be selected to the exclusion of the other side band by the filter TF2. This band will have added thereto a band extending from 3,000 cycles to zero from the circuit TL, and the two bands will be applied to the modulator Ml in the output of which will appear a lower side band comprising sub-bands from 6,000 to 9,000 cycles and from 32.000 to 6,000 cycles, the former correspending to the band from the channel TL,l

and the latter to the band. from the channel TLT The two sub-bands comprising the lower side band of ythe modulator M2 will be selected by the tilter TF3 and from this point the operation will be sameJ in the case ofiFig'fQ, as already described.

In a similar manner, the two sub-bands (if-.66,000 to 9,000 Cycles and 3,000 t 0,000 impressed'on the demodulator D2 result in the: production of a lower side band in the output circuit ot said demodulator, which side band comprises two'v sub-bands, one eX- teriding from zeroto 3,000 cycles and the other extending from 3,000 to 6,000 cycles. The tome1""sibba1id corresponds to subbalnd. N o. l andthe latter to subi-band No. Q. These two, sub1-bands will be selected to the exclusion of the trequengies in the upper side band and the sub-band No. 1 will be selected by the ilter RF2 to be impressed upon the demodulator D1, the sub-band from zeiolto 3,000 being selected into the circuit REZ. The band fr.o1n'-3,000v to 6,000 cycles, when limpressed upon the demodulator D1, appears4 in theoutput circuit'thereof as a lower side band extending from zero to 3,000 cycles, and said lower side band is selected intio the circuit RL? Various other obvious modications in the frequencies employed may .also be used. i

" It will be obvious that the general principles herein disclosedmay be embodied in many other organizationswidely dierent from thse illustrated Awithoutr departing trom the spirit ofthev invention as defined in the following claims.

` lVhatis claimediS: l

lf The method of signaling which consists' in successively stepping up a signaling band in the frequency spectrum andvcombiningWthstepped-up bandV with an additional signalingband each time a stepping u operation occurs. i Tiie methodot signaling which consists inlstepping 'up a signaling band in the frequency spectrum, combining with th 4stepped-up band another signaling band,

stepping up both bands together,'and combining with the resultant stepped-up band another vsignaling band.

` 3. The method ot signalingl which consists in stepping up a signaling band in the frequency spectrum to a'point above that occupied by another normal signaling band, combining the stepped-up band with the said other normal signaling band` stepping upv the resultant "band to a point in the trequency spectrum above a third normal signaling band, and combining the stepped-up ban'd'with said third band. i v 4. The method of signaling which consists in stepping up in the frequency spec- Ltrum a normal signaling band, combining therewithV another normal signaling band, steppingthe combined bands up in the frequency spectrum, combining with the stepped-up bands an additional normal band, and continuing the process ot stepping up'and adding normal signaling bands for all ot the signaling bands to be transmitted.

5. The method of signaling which consists in stepping up 'a normal signaling band to a point in the frequency spectrum above a second normal signaling band, combining with the stepped-up band the second normal signaling band, stepping up the combined band to a point in thefrequency spectrum above a third normal signaling band, combining the stepped-up bands with said third normal signaling band, and continuin the process ot stepping up bands and a ding normal signaling bands tor all of thenormal signaling bands to be transmitted.

6. The method of signaling which consists in modulating a normal signaling band with a carrier so chosen as to produce a side band correspending to the original signaling band and occupying a Jfrequency range above the original signaling band, selecting said side band to the exclusion ot other frequencies, combining with the selected band another normal signaling band, modulating the combined bands with a carrier so chosen as to translate the combined bands to a point in the frequency spectrum higher than a third normal signaling band, selecting the trequencies thus translated to the exclusion of other frequencies, and combining therewith said third normal signaling band.

7. The method ot' signaling which consists in modulating a normal signaling band with a carrier so chosen as to produce a side band corresponding to the original signaling band and occupying a 'frequency range above the original signaling band, selecting said side band to the exclusion o't other frequencies, combining' with the selected bandv another normal signaling band, modulating the combined bands with a carrier so chosen as to translate the combined bands to a point in the frequency spectrum higher than a third normal signaling band, selecting the trequencies thus translated to the exclusion of other frequencies, combining therewith said third normal signaling band, modulating the resultant band ot frequencies with a carrier frequency so chosen as to step up the total band ot frequencies to a point in the frequency spectrum above a tourth normal signaling band, combining the band so stepped up with said fourth signaling band andcontinuing the process ot' stepping up by modulation and combining with normalsignaling bands for each ot the normal sngnaling bands to be transmitted. i

8. The method otsig'naling which consists in transmitting a band of frequencies corresponding to a signal but elevated -in the frequency spectrum with respect to the normal signaling range, transmitting with said band other signaling bands, stepping down said first mentioned band in the frequency spectrum by a plurality of steps until it is brought into the normal signaling range, and selecting one of the other bands into an individual circuit each time the first mentioned band is stepped down until all of the bands have been selected.

9. The method of signaling which consists in transmitting a frequency range comprising a plurality of sub-bands corresponding to as many signals, each comprising originally its given range but elevated in said fre-l quency range above its normal range, stepping the frequency range down in the frequency spectrum until one of the sub-bands is brought into its normal signaling range and selecting this sub-band in its normal range to the exclusion of the other subbands. l

10. A method of signaling which consists in transmitting a frequency range comprising a plurality of sub-bands.` stepping down the `ange of frequencies in the frequency spectrum until one of the sub-bands is brought Within a desired normal range, selecting the latter sub-band in its normal range to the exclusion of the other subbands, stepping down the remaining subbands to a point in the frequency spectrum such that another of the bands is brought into its normal range, and selecting the latter band.

11. The method of signaling which consists in transmitting a frequency range comprising a plurality of sub-bands, stepping down the range of frequencies in the frequency spectrum until one of the sub-bands is brought Within a desired normal range, selecting the latter sub-band in its normal range to the exclusion of the other subbands, stepping down the remaining subi 1bands to a point in the frequency spectrum such that another of the bands is brought into its normal range, selecting the latter band and continuing the process of stepping down and selecting individual bands until all of the bands have been selected.

12. rlhe method of signaling Which consists in transmitting a. range of frequencies comprising a plurality of sub-bands, selecting one of the sub-hands, stepping the remaining sub-bands down to a point in the frequency spectrum such that one of them is brought within a desired normal range, and selecting the latter band in its normal range to the exclusion of the remaining bands.

13. The method of signaling which consists in transmitting a range of frequencies comprising a plurality of sub-bands7 selecting one of the sub-bands, stepping' doivu the remaining sub-bands in Ythe frequency spectrum to a point such that one of said sub-bands is brought into a desired normal range, selecting the latter sub-band in its normal range to the exclusion of the remaining sub-bands, stepping down the remaining sub-bands to a point in the frequency spectrum such that still another band is brought into its normal range, and selecting the latter band.

14. The method of signaling which consists in transmitting a range of frequencies comprising a plurality of sub-bands, selecting one of the sub-bands, stepping down the remaining sub-bands in the frequency spectrum to a point such that one said sub-bands is brought into a desired normal range, selecting the latter sub-bands in its normal range to the exclusion of the remaining sub-bands, stepping down the remaining sub-bands to a point in the frequency spectrum such that still another band is brought into its normal range, selecting the latter band, and continuing the process of stepping down and selecting until all of the bands have been selected.

15. The method of signaling which consists in transmitting a range of frequencies comprising a. plurality of sub-bands, modulating the range of frequencies with a carrier so chosen as to bring one of the sub-bands to a point in the frequency spectrum Within a. desired normal range, and selecting the latter sub-band in its normal range to the exclusion of the remaining subbands.

16. The method of signaling Which consists in transmitting a range of frequencies comprising a plurality of sub-bands, modulating the said range of frequencies With a carrier frequency so chosen as to translate the range of frequencies to a point in the frequency spectrum such that one of the sub-bands is brought into a desired normal range, selecting the latter sub-band in its normal range to the exclusion of the remaining sub-bands, modulating the remaining sub-bands with a carrier so chosen as to translate them to a point in the frequency spectrum such that one of the remaining sub-bands will be brought into its normal range, and selecting the latter sub-band.

17. The method of signaling which consists in transmitting a range of frequencies comprising a plurality of sub-bands, modulating the said range of frequencies with a carrier frequency so chosen as 'to translate the range of frequencies to a point in the frequency spectrum such that one of the subbands is brought into a desired normali range, selecting the latter sub-band in its norma-l range to the exclusion of the remaining sub-bands7 modulating the remaining sub-bands with a carrier frequency so chosen as to translate them to a point in the frequency spectrum such that one of the remaining sub-bands Will be brought into its normal range, selecting the latter subfband,

and continuing the process of modulating and selecting bands until all of the bands have been selected. 4

18. The method of signaling which consists in` transmitting a pluralitv of sub-bands arranged one above the other. .in the frequency spectrum, successively stepping down the sub-bands in the frequency spectrum simultaneously, and selecting one of the stepped-down bands for each step.

19. The method of signaling which consists in transmitting a plurality of sub-bands arranged one above the other in 'the frcquency spectrum, simultaneously stepping down of the bands by successive steps so as to bring one of the bands at a time into a desired normal range, and selecting the band brought into normal range from the remaining bands after each operation of stepping down.

20. In a signaling system, a plurality of signa-ling channels each including means to provide a range of signaling frequencies, means to successively step up a. range of frequencies from one of said signaling channels. and means to successively add ranges of fre-- quencies from the other signaling channels for each stepping-up operation.

21. In a signaling system, a plurality of signaling channels each including means to provide a range of signaling frequencies, means to successively step up ay range of frequencies from one of said signaling channels, and means to add successively ranges of frequencies from the other signaling channels for each steppingaip operation so that the added frequencies may be stepped up simultaneously as they are added.

22. In a signaling system, a plurality of signaling channels each including means to provide a range of signaling frequencies, means to step up a band of frequencies from oneof said signaling channels, means to combine With the stepped-up band a band of frequencies from another signaling channel, means to step up both bands together, and means to combine therewith a band from a third signaling channel.

23. In a signaling system, a plu 'ality of signaling channels each including means to provide a range of signaling frequencies, means to step up a band of frequencies from one of said signaling channels, means to combine With the stepped-up band a band .of frequencies from another signaling channel, means to step up both bands together, means to combine therewith a band from a third signaling channel, and means to successively step up the combined bands and add additional bands from other channels until the bands from all of the channels have been added.

24. In a signaling system, a plurality of signaling channels each including means to provide a range of signaling frequencies, means to modulate a band of frequencies from one of said signaling channels by a carrier so chosen as to step the band up in the frequency spectrum, means to combine Ywith the stepped-up band a band from an other sig lling channel, means to modulate the combined bands with a carrier frequency so chosen as to step up the combined bands in the frequency spectrum, and means to combine with the stepped-up bands a band from still another signaling channel.

E25. In a signaling system, a plurality of signaling channels each including means to provide a range of signaling frequencies, n'ieans to modulate a band of frequencies from one of said signaling channels by a arrier so chosen as to step the band up 'in the frequency spectrum, means to combine with the stepped-up band a band from another signaling channel, means to modulate thc combined bands with a carrier frequency so chosen as to step up the combined bands in the frequency spectrum, means to combine with the steppedaip bands a band from still another signaling channel, and means to successively step up the combined bands by n'iodulating with carrier frequencies and adding bands from other signaling channels until the bands of all of the signaling channels have been added.

26. In a signaling system, a main transmission circuit and a plurality of signa-ling channels, means to transmit a signaling band from said main transmission circuit together Withother signaling bands, means to successively step down said first mentioned signaling band in the frequency spectrum until it is brought into a desired normal range, and means to select one of the other sigi'ialing bands into one of said signaling channels for each stepping down operation until all of the bands have been selected. into the signaling channels.

27. In a signaling system. a main transmission circuit and a plurality of signaling channels, means to transmit over said main transmission circuit a range of frequencies comprising a plurality of sub-bands, means to step down said range of frequencies to a point in the frequency spectrum such that one of the sub-bands is brought into a desired normal range, means to select the latter band into one of said signaling channels. means 'to step down the remaining bands to a point such that one of them is brought into its normal range, and means to select the latter band into another sig maling channel.

28. In a signaling system. a main transmission circuit and a plurality of signaling channels, means to transmit over said main transmission circuit a range of frequencies comprising a plurality of sub-bands, means to step down said range of frequencies to a point in the frequency spectrum such that one of the sub-bands is brought into a desired normal range, means to select the latter band into one of said signaling channels, means to step down the remaining bands to a point such that one of them is brought into its normal range, means to select the latter band into another signaling channel, and means to continue the process of stepping down and selecting bands until all of the bands have been selected into their signaling channels.

29. In a signaling system, a main transmission circuit and a plurality of signaling circuits, means to transmit a range of frequencies over said main transmission circuit including a plurality of signaling bands. one of which is in its normal signaling range, means to select the latter signaling band into one of said signaling channels, means to step down the remaining signaling bands until one of them is brought into its normal signaling range, means to select the latter signalingband into another signaling channel, means to step down the unselected sub-bands until another band is brought Within its normal signaling range,

and means to select the latter sub-band into a third signaling channel.

30. In a signaling system, a main transmission circuit and a plurality of signaling circuits, means to transmit a range of frequencies over said main transmission circuit including a plurality of signaling bands, one of which is in its normal signa ing range, means to select the latter signaling band into one of said signaling channels, means to step down the remaining signaling bands until one of them is brought into its normal signaling range, means to select the latter signaling band into another signaling channel, means to step down the unselected sub-bands until another band is brought Within its normal signaling range, means to select the latter sub-band into a third signaling channel, and means 'to successively step down the remaining bands and to select the bands brought into their normal signaling range until all of the bands have been selected into their signaling channels.

In testimony whereof, I have signed my name to this specification this th day of June, 1921.

LLOYD ESPENSCHIED. 

